Problem: can't log into the backend of Joomla to use the administrator functions.

Every time I entered my credentials, the page just looped back to the login form, with no error message about a mismatch between username and password. I was, in fact, logged in, but couldn't get passed the login page!

Dead end: I spent hours trying to figure out how to repopulate jos_core_acl_aro_map, but it has been replaced by jos_core_acl_groups_aro_map. There was nothing wrong with the access control list at all.